Currently, the related manufacturers have a trend to fabricate slim, compact and portable multimedia products. The batteries of multimedia products should have long running time to benefit portability. High fidelity and high sampling rate can effectively promote the audio quality of multimedia products.
The class A amplifier is essentially used in high-end audio devices and features very low audio distortion. However, the class A amplifier has a very high power consumption. The class A amplifier uses active elements, and the elements are biased at the linear working range thereof. In other words, the bias working point is set at the nearby of the middle point of the load line. The class A amplifier is turned on in the entire cycle of signal input. When no signal is input, the class A amplifier is still turned on. Therefore, the class A amplifier has a very low power efficiency—only about 25% theoretically. Because of high power consumption, the class A amplifier needs a large heat sink to dissipate heat.
The class B amplifier uses dynamic input signals to turn on/off the transistors, and the bias working point is almost zero. When no signal is input, the output terminals almost consume none energy. Therefore, the class B amplifier has higher power efficiency—about 78.5% theoretically. Because of non-conduction of the transistors QN and QP, the class B amplifier has a non-linear working area and thus has serious crossover distortion.
The class AB amplifier works and functions between the class A amplifier and the class B amplifier. The class AB amplifier uses two diodes to vary the bias working point and eliminate the crossover distortion appeared in the class B amplifier with a penalty of having power efficiency lower than the class B amplifier. The class AB amplifier has the advantages of a lower quiescent current (lower quiescent power consumption) than the class A amplifier and a lower distortion than the class B amplifier. However, the class AB amplifier has the disadvantage of quiescent power consumption higher than the class B amplifier. Therefore, the class AB amplifier needs an additional heat sink.
The abovementioned amplifiers are all traditional linear amplifiers except the bias working points thereof are different. The class D amplifier is a switching amplifier completely distinct from the traditional linear amplifiers.
In comparison with the linear amplifiers, the class D amplifier has very high power efficiency—100% theoretically. Because of high power efficiency, the class D amplifier generates less heat, and none additional heat sink is needed. Therefore, the class D amplifier can be economically manufactured and is widely used.
Refer to FIG. 1 for a conventional class D amplifier. The conventional class D amplifier is very suitable for compact and portable electronic products. The class D amplifier comprises a modulation circuit 1, an amplifier circuit 2 and a low pass filter 3. The modulation circuit 1 usually has the PWM (Pulse Width Modulation) function and the SDM (Sigma Delta Modulation) function. The modulation circuit 1 converts an input audio signal 5 into a pulse width-based two-level voltage signal 6, and the pulse width-based two-level voltage signal 6 is used to control the turn-on/turn-off of the amplifier circuit 2. Then, the signal is restored by the low pass filter 3 and output by a speaker 4.
The input audio signal 5 is converted by the modulation circuit 1 into the pulse width-based two-level voltage signal 6, and the pulse width-based two-level voltage signal 6 is amplified by the amplifier circuit 2. After amplified, the signal is still a two-level voltage signal 6. The low pass filter 3 filters out the high-frequency harmonic and decreases the affection of noise and electromagnetic interference. In the time domain, the low pass filter 3 functions like an integrator, gradually accumulating or releasing the signal levels or signal energy to restore the modulated signal.
The two-level voltage signal 6 has very great instantaneous voltage difference. Thus, the low pass filter 3 is hard to accumulate energy synchronously, and the signals are likely to have a phase difference. Therefore, the output signal has distortions 7 and cannot be restored into the audio voltage signal in high fidelity and low distortion. Compared with the sinusoidal input audio signal 5, the two-level voltage signal 6 has many distortions 7.